The field of the invention is electron paramagnetic resonance (EPR) and magnetic resonance (MR) spectroscopy, and particularly, the disposition of samples to be examined in such spectroscopy systems.
Electron paramagnetic resonance spectroscopy is conducted to study electrons which are in a paramagnetic state and which is called electron paramagnetic resonance (EPR) or electron spin resonance (ESR). In electron paramagnetic resonance spectroscopy a sample to be investigated is subjected to a polarizing magnetic field and one or more radio frequency magnetic fields. The frequency, strength, direction, and modulation of the applied magnetic fields vary considerably depending upon the particular phenomena being studied. Apparatus such as that disclosed in U.S. Pat. Nos. 3,358,222 and 3,559,043 have been employed for performing such experiments in laboratories. Samples which are the subject of the EPR measurement are placed in a microwave resonator where they are subjected to the RF magnetic field. The microwave resonator may take the form of a cavity resonator such as that disclosed in U.S. Pat. Nos. 3,931,569 and 3,757,204, or it may be a loop-gap resonator such as that disclosed in U.S. Pat. No. 4,446,429. A major objective of the resonator is to enhance the RF magnetic field throughout the extent of the sample.
Loop-gap resonators (LGR) have become a preferred resonator geometry for experiments at frequencies below X band. Cavity resonators are generally preferred at higher frequencies to about 100 GHz, with Fabry-Perot resonators preferred at ultrahigh frequencies. Both LGRs and cavity resonators are in common use at X-band (10 GHz), Q-band (35 GHz) and S-band (3 GHz), which are by far the most widely used frequency for EPR experiments. The reason for these preferences is primarily convenience. Cavity resonators are awkwardly large at S band, LGRs become extremely small at Q band, and cavity resonators are, in turn, too small to handle easily at ultrahigh frequencies.
A benefit of LGRs is that the length to diameter ratio of the sample-containing loop is typically about five, resulting in a relatively uniform microwave field over the sample. This is a substantial benefit in experiments using line samples that extend through the resonator, since all portions of the sample respond in the same way to the incident microwave field. For cavity resonators on the other hand, the microwave field varies cosinusoidally over the sample, with the number of half cycles of variation determined by the selected index of the microwave resonant mode—usually one half cycle.
Recently, uniform field (UF) microwave cavity resonance modes for use in EPR spectroscopy were introduced. These modes consist of three sections, a central section in which the fields are uniform in the dimension corresponding to the axis of the section, and two end sections that are each effectively ¼ wavelength long. Three ways were found to design the end sections: filling with dielectric, making them oversize, or making them re-entrant. A rectangular TE102 UF (TEU02) resonator with dielectric end sections and an inserted aqueous sample cell is illustrated in FIG. 2. This structure is described in detail in copending U.S. patent application Ser. No. 10/200,885 filed on Jul. 23, 2002 and entitled “Cavity Resonator Having Axially Uniform Field”.
The earliest discussion of how aqueous samples are mounted in EPR resonators considered the rectangular TE102 cavity resonator, which has a central nodal plane at the sample position where the RF electric field is zero and the RF magnetic field is a maximum. A so-called “flat cell” was employed that constrained an aqueous sample in a thin slab lying in this plane, and obtained improved performance relative to use of a sample contained in cylindrical capillary.
It has been observed by several workers using the rectangular TE102 cavity that when the flat cell is rotated a few degrees, the resonator Q-value becomes very poor, but if it is rotated precisely 90°, the Q-value recovers and good EPR signals can be obtained that are of similar intensities to signals obtained in the “parallel” orientation. A model to explain this surprising result was described by J. S. Hyde “A New Principle For Aqueous Sample Cells For EPR,” Rev. SCI. Instrum. 43 (1974) 629–631, and he also disclosed insertion of more than one flat cell in this “perpendicular” orientation. Thus, it was established 30 years ago that there are two fundamental physical principles that govern aqueous sample cell geometries in EPR spectroscopy: placement in electric field nodes and surface orientation perpendicular to E. While this qualitative fact has been known for many years, no theoretical analysis has been performed which yields quantitative facts that enable this discovery to be exploited in a practical and optimal way.